Bacterial Flagellum: Definition, Types and Composition of These Unique Structures

It is a filamentous structure that serves to drive the bacterial cell.

A flagellum is an eyelash-like appendage that protrudes from the cell body of certain bacterial and eukaryotic cells.

The main role of the flagellum is locomotion, but it often functions as a sensory organelle as well, being sensitive to chemicals and temperatures outside the cell.

The similar structure in archaea works in the same way, but it is structurally different and has been named archaellum .

Flagella are organelles defined by function rather than structure. The flagella vary greatly.

Both prokaryotic and eukaryotic flagella can be used for swimming, but they differ greatly in protein composition, structure, and mechanism of propulsion. The Latin word scourge means whip.

An example of a flagellated bacterium is the ulcer-causing Helicobacter pylori, which uses multiple flagella to propel itself through the mucous lining and reach the stomach lining.

An example of a eukaryotic flagellate cell is the mammalian sperm cell, which uses its flagellum to propel itself through the female reproductive tract.

Eukaryotic flagella are structurally identical to eukaryotic cilia, although distinctions are sometimes made according to function or length.

The fimbriae and pili are also thin appendages, but they have different functions and are generally smaller.


So far three types of flagella have been distinguished: bacterial, archaeal and eukaryotic.

The main differences between these three types are:

Bacterial flagella are helical filaments, each with a rotating motor at its base that can rotate clockwise or counterclockwise. They provide two of several types of bacterial motility.

Archaeal (archaella) flagella are superficially similar to bacterial flagella, but are different in many details and are considered non-homologous.

Eukaryotic flagella (those of animal, plant, and protist cells) are complex cellular projections that move from side to side.

Eukaryotic flagella are classified along with eukaryotic motile cilia as undulipodia to emphasize their distinctive wavy appendix role in cell function or motility.

The primary cilia are immobile, and are not undulipodia; they have a structurally different 9 + 0 axoneme in place of the 9 + 2 axoneme found in both flagella and ciliary undulipodia.


Structure and composition

The bacterial flagellum is composed of the flagellin protein . Its shape is a hollow tube 20 nanometers thick.

It is helical and has a sharp curve just outside the outer membrane; this “hook” allows the propeller shaft to point directly from the cell.

A shaft runs between the hook and the basal body, passing through protein rings in the cell membrane that act as bearings.

Gram-positive organisms have two of these basal body rings, one in the peptidoglycan layer and one in the plasma membrane.

Gram-negative organisms have four such rings: the L ring associates with lipopolysaccharides, the P ring associates with the peptidoglycan layer, the M ring is embedded in the plasma membrane, and the S ring is directly attached to the membrane. plasmatic.

The filament ends with a protective protein.

The flagellar filament is the long helical screw that powers the bacteria when the motor rotates through the hook.

In most of the bacteria that have been studied, including Gram-negative Escherichia coli, Salmonella typhimurium, Caulobacter crescentus, and Vibrio alginolyticus, the filament is composed of 11 protofilaments approximately parallel to the axis of the filament.

Each protofilament is a series of tandem protein chains. However, Campylobacter jejuni has seven protofilaments.

The basal body has several features in common with some types of secretory pores, such as the hollow rod-like “plug” at its centers that extends through the plasma membrane.

Given the structural similarities between bacterial flagella and bacterial secretory systems, bacterial flagella may have evolved from the type 3 secretion system.

However, it is not known for sure whether these pores are derived from bacterial flagella or the bacterial secretory system.


The bacterial flagellum is driven by a rotary motor (Mot complex) composed of protein, located at the anchor point of the flagellum on the inner cell membrane.

The motor is driven by the motive force of the proton, that is, by the flow of protons (hydrogen ions) through the bacterial cell membrane due to a concentration gradient created by the metabolism of the cell (Vibrio species have two flagella types, lateral and polar.

Some are powered by a sodium ion pump rather than a proton pump). The rotor carries protons through the membrane and spins in the process.

The rotor can only operate at 6,000 to 17,000 rpm, but with the flagellar filament attached it generally only reaches 200 to 1000 rpm.

The direction of rotation can be changed by the flagellar motor switch almost instantly, caused by a slight change in the position of a protein, FliG, on the rotor.

The scourge consumes a lot of energy and uses very little energy. The exact mechanism for generating torque is still poorly understood.

Because the flagellar motor has no on-off switch, the epsE protein is used as a mechanical clutch to disconnect the motor from the rotor, thereby stopping the flagella and allowing bacteria to remain in one place.

The cylindrical shape of the flagella is suitable for the locomotion of microscopic organisms; These organisms operate at a low Reynolds number, where the viscosity of the surrounding water is much more important than its mass or inertia.

The speed of rotation of the flagella varies in response to the intensity of the proton’s motive force.

This allows certain forms of speed control, and also allows some types of bacteria to reach remarkable speeds in proportion to their size; some reach approximately 60 cell lengths per second.

At such speed, a bacterium would take around 245 days to cover 1 km; Although it seems slow, the perspective changes when the concept of scale is introduced.

Compared to macroscopic life forms, it is very fast when expressed in terms of the number of bodies per second. A cheetah, for example, only achieves about 25 body lengths per second.

By using its flagella, E. coli is able to move rapidly toward attractants and away from repellants, using a random skewed walk.

With ‘corridas’ and ‘tumbos’ provoked by turning his scourge counterclockwise and clockwise, respectively.

The two directions of rotation are not identical (with respect to the movement of the flagellum) and are selected by a molecular switch.


During flagellar assembly, components of the flagellum pass through the hollow nuclei of the basal body and the nascent filament. During assembly, protein components are added at the flagellar tip rather than at the base.

In vitro, flagellar filaments spontaneously assemble in a solution containing purified flagellin as the only protein.


At least 10 protein components of the bacterial flagellum share homologous proteins with the type three secretion system (TTSS), therefore one likely evolved from the other.

Because the type three secretion system has a similar number of components as a flagellar apparatus (approximately 25 proteins), which one developed first is difficult to determine.

However, the flagellar system appears to involve more proteins overall, including various regulators and chaperones, so it has been argued that flagella evolved from a type three secretion system.

However, it has also been suggested that the flagellum may have evolved first or the two structures evolved in parallel.

The need for motility (mobility) of early single-celled organisms helps the more mobile flagella to be selected by evolution first, but the type 3 secretion system evolving from the flagellum can be viewed as a ‘reductive evolution’ and is not supported topological phylogenetic trees.

The hypothesis that the two structures evolved separately from a common ancestor explains the protein similarities between the two structures, as well as their functional diversity.

Flagella and the debate on intelligent design

Some authors have argued that flagella cannot have evolved because they can only function properly when all proteins are in place.

In other words, the flagellar apparatus is “irreducibly complex.” This has long been discredited, because many proteins can be deleted or mutated and the scourge still works, albeit sometimes with reduced efficacy.

For example, several mutations have been found that increase the motility of E. coli. Additional evidence for the evolution of bacterial flagella includes:

The existence of vestigial flagella, intermediate flagella forms, and patterns of similarities between flagellar protein sequences, including the observation that almost all core flagellar proteins have known homologies to non-flagellar proteins.

In addition, several processes have been identified that play important roles in flagellar evolution, including self-assembly of simple repetitive subunits, duplication of genes with subsequent divergence, recruitment of elements from other systems (“molecular tinkering”), and recombination.

Flagellar Arrangement Schemes

Different species of bacteria have different numbers and arrangements of flagella. Monotonous bacteria have only one flagellum (eg, Vibrio cholerae).

Photophrenic bacteria have multiple flagella located in the same place on bacterial surfaces that work together to drive bacteria in only one direction.

In certain large forms of Selenomonas, more than 30 individual flagella are arranged outside the cell body, forming a thick structure (easily visible under the light microscope) called a fasciculus.

Spirochetes, in contrast, have flagella that arise from opposite poles of the cell, and are located within the periplasmic space as shown by rupturing the outer membrane and more recently by electron cryography microscopy.

The rotation of the filaments relative to the cell body causes all bacteria to move forward with a corkscrew-like motion, even through material viscous enough to prevent the normally flagellated bacteria from passing through.

The counterclockwise rotation of a monotonous polar flagellum pushes the cell forward with the flagellum behind, like a corkscrew moving inside the cork.

In fact, water on the microscopic scale is highly viscous, very different from our daily experience of water.

The flagella are left-handed helices, and they cluster and rotate only when turned counterclockwise.

When some of the rotors reverse direction, the flagella unwind and the cell begins to “fall over.”

Even if all the flagella rotated clockwise, it is likely that they will not form a bundle, due to geometric and hydrodynamic reasons.

Such a “drop” can occasionally occur, leading to the cell seemingly shifting into place, resulting in reorientation of the cell.

The clockwise rotation of a flagellum is suppressed by cell-friendly chemicals (eg, food), but the motor is very adaptable to this.

Therefore, when moving in a favorable direction, the concentration of the chemical attractant increases and the “drops” are continually suppressed.

However, when the direction of cell movement is unfavorable (such as away from a chemical attractant), the turns are no longer suppressed and occur much more frequently, with the possibility of the cell reorienting in the direction correct.

Polar flagella are constitutively expressed and provide mobility in bulk fluid, whereas lateral flagella are expressed when polar flagella encounter too much resistance to rotate.

These provide swarming motility on surfaces or in viscous fluids.


The archaellum possessed by some archeae is superficially similar to the bacterial flagellum; In the 1980s, they were thought to be homologous on the basis of morphology and general behavior.

Both the flagellum and archaella consist of filaments that extend outside the cell and rotate to power the cell. Archaeal flagella have a unique structure that lacks a central channel.

Similar to bacterial type IV pilins, archeal flagellins (archaellins) are made with class 3 signal peptides and are processed by an enzyme similar to type IV prepilin peptidase.

Archaellins are typically modified by the addition of N-linked glucans, which are necessary for proper assembly or function.



In order to emphasize the distinction between the bacterial flagella and eukaryotic cilia and flagella, some authors attempted to replace the name of these two eukaryotic structures with “undulipodia.”

For example, all Margulis articles since the 1970s, or “cilia” for both, eg, Hülsmann, 1992; Adl et al., 2012; most of Cavalier-Smith’s work, preserving “flagella” for bacterial structure.

However, the discriminative use of the terms “cilia” and “flagella” for eukaryotes adopted in this article remains common (eg, Andersen et al., 1991; Leadbeater et al., 2000).

Internal structure

In addition to the axoneme and the basal body, which are relatively constant in morphology, other internal structures of the flagellar apparatus are: the transition zone, where the axoneme and the basal body join.

And the root system (microtubular or fibrillar structures that extend from the basal bodies to the cytoplasm), more variable and useful as indicators of the phylogenetic relationships of eukaryotes.

Other, rarer structures are the paraflagular (or paraxial, paraxonemal) rod, the R fiber and the S fiber: 63-84 For superficial structures, see below.

Flagella vs cilia

The regular beat patterns of eukaryotic cilia and flagella generate movement at the cellular level.

Examples range from the propulsion of individual cells, such as sperm swimming, to the transport of fluid along a stationary layer of cells, such as in the respiratory tract.

Although eukaryotic flagella and motile cilia are ultrastructurally identical, the beat pattern of the two organelles may be different.

In the case of flagella, the movement is often flat and wavy, while motile cilia often perform a more complicated three-dimensional movement with a power stroke and recovery.

Intraflagellar transport

Intraphylalic transport, the process by which axonemal subunits, transmembrane receptors, and other proteins move up and down along the flagellum, is essential for the proper functioning of the flagellum, both in motility and transduction of signs.

Evolution and occurrence

Eukaryotic flagella or cilia, probably an ancestral feature, have become widespread in almost all groups of eukaryotes, as a relatively perennial condition, or as a stage of the flagellate life cycle (e.g., Zoids, gametes, zoospores, which can be produced continuously or not).

The former is found in specialized cells of multicellular organisms (eg, the choanocytes of sponges or the ciliated epithelium of metazoans), as in ciliates and many eukaryotes with a “flagellate condition” (or “monadoid level of organization”).

The stages of the flagellate life cycle are found in many groups, for example, many green algae (male gametes and zoospores), bryophytes (male gametes), pteridophytes (male gametes).

Some gymnosperms (cycads and Ginkgo, as male gametes), centric diatoms (male gametes), brown algae (zoospores and gametes), oomycetes (asexual zoospores and gametes), hypokites (zoospores), labyrinthulomycetes (zoospores).

Some apicomplexes (gametes), some radiolaria (probably gametes), foraminifera (gametes), plasmodiophoromycetes (zoospores and gametes), myxoases (zoospores), metazoa (male gametes), and chytrid fungi (zoospores and gametes).

Flagella or cilia are completely absent in some groups, probably due to a loss rather than being a primitive condition.

Loss of cilia occurred in red algae, some green algae (Zygnematophyceae), gymnosperms except cycads and ginkgo, angiosperms, pennate diatoms, some apicomplexians, some amoebozoans, in the sperm of some metazoans and in fungi (except chytrid).


Various terms related to flagella or cilia are used to characterize eukaryotes. Depending on the surface structures present, the flagella can be:

Whiplash flagella (= smooth acronematic flagella) : hairless, for example in Opisthokonta.

Hairy flagella (= foam, flimmer, pleuronematic flagella) : with hairs (= mastigonemas sensu lato), divided into:

With fine hairs (= non-tubular, or simple hairs) : occurs in Euglenophyceae, Dinoflagellata, some Haptophyceae (Pavlovales).

With stiff hairs (= tubular hairs, retronemas, mastigonemes sensu stricto), divided into:

Bipartite hairs : with two regions. It occurs in Cryptophyceae, Prasinophyceae, and some Heterokonta.

Tripartite hairs (= straminipilous) : with three regions (a base, a tubular axis and one or more terminal hairs). It occurs in most of Heterokonta.

Pantonematic flagella : with a single row of hairs.

Pantonematic flagella : with two rows of hairs.

Acronematics : flagella with a single terminal mastigonema or flagellar hair (eg bodonids); some authors use the term as a synonym for whiplash.

With scales : for example, Prasinophyceae.

With thorns : for example, some brown algae.

With wavy membrane : for example, some kinetoplastids, some parabasalids.

With proboscis (protrusion of the cell in the form of a trunk) : such as apusomonas, some bodonides.

Depending on the number of flagella, the cells may be (remembering that some authors use “ciliates” instead of “flagellates”:

Uniflagellate : for example, most Opisthokonta.

Biflagellates : for example, all Dinoflagellata, the gametes of Charophyceae, most bryophytes, and some metazoans.

Triflagellate : for example, the gametes of some foraminifera.

Quadriflagellates : such as some prasinophyceae, collodictyonidae.

Octoflagellates : for example, some diplomonada, some prasinophyceae.

Multiflagellate : for example, opalinata, ciliophora, stephanopogon, parabasalida, hemimastigophora, caryoblastea, multicilia, the gametes (or zoids) of oedogoniales (chlorophyta), some pteridophytes and some gymnosperms.

According to the place of insertion of the flagellum:

Opisthokonta : cells with flagella inserted posteriorly, for example in Opisthokonta (Vischer, 1945). In Haptophyceae, flagella insert lateral to terminal, but are directed posteriorly during rapid swimming.

Akrokont : cells with apically inserted flagella.

Subakrokont : cells with flagella inserted subapically.

Pleurokont : cells with laterally inserted flagella.

According to the pattern of strokes:

Slip : a scourge that crawls on the substrate.

Heterodynamic : flagella with different beat patterns (usually with one flagellum working in catching food and the other working in planning, anchoring, propulsion, or “steering”).

Isodynamic : flagella striking with the same patterns.

Other terms related to the flagellar type:

Isokont : cells with flagella of equal length. It was also previously used to refer to the Chlorophyta.

Anisokont – cells with flagella of unequal length, for example some Euglenophyceae and Prasinophyceae.

Heterokont : term introduced by Luther (1899) to refer to the Xanthophyceae, due to the pair of flagella of unequal length.

It has acquired a specific meaning when referring to cells with an anterior straminipilous flagellum (with tripartite mastigonemes, in one or two rows) and a generally smooth posterior flagellum. It is also used to refer to the taxon Heterokonta.

Stephanokont – cells with a crown of flagella near their anterior end, for example, the gametes and spores of Oedogoniales, the spores of some Bryopsidales. Term introduced by Blackman & Tansley (1902) to refer to the Oedogoniales.

Akont : cells without flagella. It was also used to refer to taxonomic groups, such as Aconta or Akonta: the Zygnematophyceae and Bacillariophyceae (Oltmanns, 1904), or the Rhodophyceae (Christensen, 1962).